Other Options
1 Get a custom built 'tuned' exhaust system by any number of custom exhaust shops. Any of the above looks are then possible. The one I did not see was the squircle (square/circle) shape like the angle eyes in the zenons or ovals.

2 just increase the size of the pipes as this will change the resonance in the back box. Bigger pipes = depth in sound. Look at ovals pipes to generate volume. Not a lot of cost. Have twin rear tips if you like